1. Field of the Invention
The present invention is related to digital radar systems, and more particularly, to analysis of received signals to detect and eliminate the display of echo signals caused by birds.
2. Description of the Related Art
Radar systems used for air traffic control as well as those used for other purposes receive echo signals from many objects other than the desired targets, i.e., airplanes. There is "ground clutter" from trees, buildings and other land structures as well as echo signals from other objects in the air, particularly, from birds. In a conventional radar system, displaying targets corresponding to "bird echoes" is avoided by setting the amplitude threshold for displayed targets higher than the amplitude of signals received from birds. In addition, birds having uniform movements, such as flocks of geese, can be "edited" based upon the speed of movement from one scan to the next.
However, the conventional techniques are incapable of distinguishing between an aircraft having a small cross section and a dense group of birds. The problem is aggravated when the target has a low angle of elevation so that editing of the echo signals is performed on signals output from a Doppler filter, such as a moving target indicator (MTI), used to remove ground clutter. The signals output by a Doppler filter typically have an uneven gain versus velocity characteristic which makes the setting of the threshold virtually impossible. As a result, either bird echoes are displayed or some aircraft are not displayed, or both.
Some attempts have been made to edit bird echos using, e.g., bursts of pulses with a constant interpulse period during each burst and changes in the interpulse period from one burst to the next. However, the beam width of conventional radar is typically too small to provide a sufficient number of different interpulse periods to resolve ambiguities in range rate. As a result, it is difficult to satisfactorily distinguish the range rate of birds from the range rate of an aircraft.